The field of immediate interest in the present application is that of over-the-road trucking. The economies of trucking dictate that fuel economy should be an important consideration. It is significant not only for large freight line companies owning huge fleets of trucks, but also for the single operator. Much effort has been expended in enhancing the fuel economy of automotive engines including, for example, making the vehicles more aerodynamic, increasing engine efficiencies, and reducing emissions.
The present invention is concerned with the requirement for providing forced air through the radiator for cooling the engine, air conditioning system, intercooler and the like. It is well known that a fan is, at times, needed for cooling and, at other times, unnecessary. It is also well known that the fan can be a major consumer of horsepower, often on the order of 55 horsepower, which will affect vehicle performance. Since horsepower is directly related to the speed cubed, it is desirable to operate the fan at the lowest available fan speed to minimize horsepower consumption. Similarly, it is undesirable to operate the fan when cooling is unnecessary or to overcool the engine. As such, a significant improvement in fuel economy and engine performance may be achieved if the fan is operated only when needed.
It has been found that the smaller pulley ratios have been unable to adequately cool the engine operating at lower engine speeds (typically about 1100 rpm). In order to solve the low fan speed problem and to properly cool the engine, it has been necessary to increase the fan to engine speed drive ratio from about 1:1 to 1.2-1.4:1. The system has been designed to meet the maximum heat rejection at the low engine speeds such as, for example, at about 1100 rpm. Thus, for engine speeds of about 1100 rpm, the fan will rotate at a speed capable of meeting the maximum heat rejection and necessary cooling requirements.
While that might, on the surface, solve the problem of fan speed at low engine speeds, it creates other problems when the engine operates at higher speeds. There can be significant periods of time when the truck is climbing or descending hills and the like, and where the vehicle is operated for extended periods of time at the higher engine speeds. By gearing down, a higher torque is achieved for climbing hills and the like. However, when the pulley ratio is simply changed to boost the fan speed at low engine speeds, the fan will be substantially overspeeded when the engine speed is increased as for hill climbing and the like. Indeed, the increased drive ratio may even attempt to operate the fan above its maximum rated speed. Moreover, operating the fan in the overspeed mode overcools the engine and unnecessarily increases engine horsepower draw and fuel waste.
Attempts of prior fan clutches to either reduce the speed of the fan or declutch the fan in conditions when it is not needed have not been entirely successful. Approaches utilizing dry clutches have typically resulted in on/off operation since the dry clutch could not slip for long without overheating. Inherent in on/off applications is the typical shock load to the drive unit when the drive clutch is engaged. The shock load is not only undesirable from the viewpoint of loading and wear on the mechanism, but is also aesthetically detrimental. When the vehicle is parked, for example, but the engine is running in order to maintain heat or cooling, the fan clutch will typically cycle on and off, creating significant audible disturbance. A further disadvantage of on/off operation is that the system is effectively a coolant temperature loop control system. This introduces a response time delay and the clutch mechanism is incapable of dynamically responding to engine conditions to insure that the fan operates at precisely the desired speed and/or to selectively determine fan speed.
One attempt to avoid the problems with dry clutch fan drives has been the attempted use of viscous coupling between the input and output members of the drive unit. Unfortunately, these approaches have also had their drawbacks. First, viscous couplings have poor release capability and no "lock-up" capability so that the drive input and output members may not be driven at the same speed. Moreover, fan drives using viscous couplings have limited horsepower capability, and cannot quickly dissipate heat from the engine. Most viscous coupling designs are slow to engage after sensing heat, and cannot be completely declutched when cooling is not desired.
Wet clutch mechanisms for driving engine fans have also been used. Wet clutch mechanisms, which typically use oil in the engine sump, have been used to provide relatively continuously variable speed, and will not overheat under most conditions by virtue of the oil-bathed clutch mechanism. If space were not a problem, it would be relatively straightforward to provide a continuously variable relatively reliable clutch mechanism to couple the fan and engine. However, when one appreciates the desires of the truck and engine designers to minimize the space requirements "under-the-hood" and the critical need to efficiently use the under-the-hood space, it will be quickly appreciated that a relatively small envelope is available for the clutch mechanism. The envelope is limited axially by the distance between the radiator and the engine, and it is limited radially, as a practical matter, by the size of sheave which can be accommodated for the pulley driving the fan.
In engines having relatively wide operating rpm ranges, and therefore wide operating oil pressure ranges, the clutch mechanism must have a relatively large hydraulic piston operating area to reliably operate the clutch at the oil pressure extremes, that is, from relatively low oil pressures at idle to relatively high oil pressures at high engine speeds. Similarly, they have required relatively bulky mechanisms to pump or pressurize the oil in the clutch housing As a result, wet clutches with adequate horsepower for fan drive operation have been relatively large.
In some applications, such as off-the-road vehicles including tractors, loaders, graders and the like, there is adequate room in the engine compartment to tolerate the relatively large clutch mechanisms typically associated with wet clutches. However, in other applications where space requirements are more critical, including, for example, over-the-road tractors, the requirements for aerodynamics, appearance, vehicle size, vehicle weight and the like have all combined to reduce the size of the engine compartment. Thus, the relatively large wet clutches having adequate horsepower for fan operation are less compact than desired and may pose a problem for such applications.